Step 1Heat the oven to 400 degrees and adjust a rack on the topmost position.
Step 2Prepare a stovetop steamer to cook the potatoes. Steam the potatoes until they are soft all the way through, 30 to 40 minutes.
Step 3While they're still hot, put the potatoes, skins and all, into a stand mixer with a paddle attachment. Mix on medium speed until the potatoes are just broken up, 10 to 15 seconds. Don't overmix, or the potatoes will become gluey. Take one-half cup Atomic horseradish sauce and squeeze it in your fist to squeeze out all the liquid. Add this to the potatoes and mix on medium speed just to combine, about 10 seconds. Again, don't overmix. Add 1 tablespoon kosher salt and one-half cup crème fraîche, and mix again until just barely combined, about 10 seconds.
Step 4Set aside until the potatoes are just warm. In the meantime, beat 2 egg yolks with 1 egg lightly with a fork.
Step 5When the potatoes are just warm (so the eggs do not cook when they are incorporated), add the beaten eggs and pulse the mixer on and off a couple of times until the eggs are just mixed in. Detach the bowl from the machine and finish incorporating the egg mixture with a rubber spatula.
Step 6Put the potato mixture into a large zip-lock bag and cut a one-half-inch hole in one corner.
Step 7Line a baking sheet with parchment paper and spray the paper with cooking spray.
Step 8Pipe the potato into 11/2-inch round rosettes, leaving about 2 inches between each; the egg will cause them to puff up a bit, so they need the space to grow. Brush each rosette with melted butter, using about half of the butter. At this point, the potatoes can be refrigerated until ready to bake; if they are refrigerated, keep in mind that they will need a little longer to bake through, as they will be cold.
Step 9Put the potatoes in the oven and bake for 15 minutes, turning the baking sheet halfway through cooking time for even cooking. Increase the oven temperature to 500 and bake the potatoes for 12 to 15 minutes more, turning halfway through, until they are golden brown.
Step 10Remove the potatoes from the oven. Brush them with the remaining butter, sprinkle with Maldon sea salt and top each rosette with a generous grating of fresh horseradish and finely diced chives.